The present invention relates to processes for isomerising aromatic compounds containing eight carbon atoms. More particularly, the invention relates to a process for isomerising aromatic compounds containing eight carbon atoms in which the catalyst used containing a zeolite with structure type EUO and an element from group VIII of the periodic table (Handbook of Chemistry and Physics, 1964-1965).
Different zeolites corresponding to structure type EUO have been described in the prior art, examples being EU-1 zeolite described in European patent EP-A-0 042 226, TPZ-3 zeolite described in EP-A-0 051 318 and ZSM-50 zeolite described in U.S. Pat. No. 4,640,829.
More specifically, the present invention provides a process for isomerising aromatic compounds containing eight carbon atoms, in which a catalyst is used which contains at least one zeolite with structure type EUO and at least one metal or compound of a metal from group VIII and in which at least one compound with a boiling point of about 80xc2x0 C. to about 135xc2x0 C. other than the aromatic compounds containing eight carbon atoms is introduced into the reaction zone with the feed containing the compound or compounds to be isomerised and the hydrogen necessary for the reaction. The group VIII metal is normally selected from noble metals or compounds of noble metals from group VIII and in particular, platinum or palladium is used, or a compound of at least one of these metals, preferably platinum or a platinum compound.
The process of the present invention has a number of advantages over the prior art, including: a reduction in the loss of aromatic compounds containing eight carbon atoms through secondary side reactions of dismutation, transalkylation, hydrogenation and cracking.
In known processes for isomerising aromatic compounds containing eight carbon atoms, a feed which is generally low in para-xylene with respect to the thermodynamic equilibrium of the mixture (i.e., in which the amount of para-xylene is substantially lower than that of the mixture at thermodynamic equilibrium at the temperature under consideration, this mixture comprising at least one compound selected from the group formed by meta-xylene, ortho-xylene, para-xylene and ethylbenzene), and generally rich in ethylbenzene with respect to this same mixture at thermodynamic equilibrium is introduced into a reactor containing at least one catalyst, under suitable pressure and temperature conditions to obtain from the reactor outlet a composition of aromatic compounds containing eight carbon atoms which is as close as possible to the composition of said mixture at thermodynamic equilibrium at the temperature of the reactor.
The para-xylene and optionally ortho-xylene, which are the desired isomers as they are of great importance in particular for the synthetic fibre industry, are then separated from this mixture. The meta-xylene and ethylbenzene can then be recycled to the isomerisation reactor inlet so as to increase the production of para-xylene and ortho-xylene. When ortho-xylene is not to be recovered, it is recycled with the meta-xylene and ethylbenzene.
However, there are a number of problems associated with isomerising aromatic compounds containing eight carbon atoms per molecule, caused by secondary reactions. Thus in addition to the principal isomerisation reaction, hydrogenation reactions are observed, such as hydrogenation of aromatic compounds to naphthenes, and naphthene ring opening reactions which lead to the formation of paraffins containing at most the same number of carbon atoms per molecule as the naphthenes from which they are formed. Cracking reactions are also observed, such as paraffin cracking which leads to the formation of light paraffins, typically containing three to five carbon atoms per molecule, also dismutation and transalkylation reactions which lead to the production of benzene, toluene, aromatic compounds containing nine carbon atoms per molecule (for example trimethylbenzenes) and heavier aromatic compounds.
All together, those secondary reactions are highly deleterious to the yields of the desired products.
The quantity of secondary products formed (primarily naphthenes typically containing 5 to 8 carbon atoms, paraffins typically containing 3 to 8 carbon atoms, benzene, toluene, aromatic compounds containing 9 or 10 carbon atoms per molecule) depends on the nature of the catalyst and the operating conditions of the isomerisation reactor (temperature, partial pressures of hydrogen and hydrocarbons, feed flow rate).
In conventional processes for isomerising aromatic compounds containing eight carbon atoms, a mixture of xylenes and ethylbenzene is brought into contact with a suitable catalyst, generally containing at least one group VIII noble metal and a zeolite, in order to bring the mixture of aromatic compounds containing eight carbon atoms to a composition which is as close as possible to the composition corresponding to thermodynamic equilibrium at the temperature under consideration.
The Applicant has now discovered that, surprisingly, a process for isomerising aromatic compounds containing eight carbon atoms in which a catalyst is used which contains at least one zeolite with a structure type EUO and at least one metal or a compound of a metal from group VIII, and in which at least one compound with a boiling point of about 80xc2x0 C. to about 135xc2x0 C. other than the aromatic compounds containing eight carbon atoms is introduced into the reaction zone with the feed containing the compound or compounds to be isomerised and the hydrogen necessary for the reaction, has much better performances than in prior art processes.
In the process of the present invention, the catalyst contains at least one zeolite with structure type EUO, and at least one metal or compound of a metal from group VIII, and thus has the advantage of being carried out in a lower temperature range, at lower partial pressures of hydrogen and at higher HSVs (weight of feed/weight of catalyst/hour).
Preferably, a catalyst is used which contains a zeolite with structure type EUO, for example EU-1 zeolite and platinum. This catalyst contains at least one zeolite with structure type EUO, at least partially in its acid form, this zeolite comprising silicon and at least one element T selected from the group formed by aluminium, iron, gallium and boron, preferably aluminium and boron, and in which the global Si/T atomic ratio is about 5 to 100, preferably about 5 to 80, and more preferably about 5 to 60. This catalyst also comprises at least one matrix (or binder), and at least one metal or compound of a metal from group VIII of the periodic table. This catalyst also optionally comprises at least one metal or compound of a metal selected from the group formed by metals or compounds of metals from groups IIIA and IVA of the periodic table, and optionally sulphur or at least one sulphur compound. The metal is usually selected from the group formed by tin and indium.
The matrix is generally selected from the group formed by natural clays (for example kaolin or bentonite), synthetic clays, magnesia, aluminas, silicas, silica-aluminas, titanium oxide, boron oxide, zirconia, aluminium phosphates, titanium phosphates, and zirconium phosphates, preferably from elements of the group formed by aluminas and clays. This matrix may be a simple compound or a mixture of at least two of these compounds.
The zeolite with structure type EUO is at least partially, preferably almost completely, in its acid form, i.e., in its hydrogen (H+) form.
The weight content of this zeolite is 1% to 90%, preferably 3% to 60%, more preferably 4% to 40%, with respect to the total catalyst weight.
This catalyst also contains at least one matrix, or binder, providing the complement to 100% in the catalyst.
The process for isomerising aromatic compounds containing eight carbon atoms of the present invention is carried out at a temperature of about 300xc2x0 C. to 500xc2x0 C., preferably about 320xc2x0 C. to 450xc2x0 C., and more preferably about 350xc2x0 C. to 420xc2x0 C., at a partial pressure of hydrogen of about 0.3 to 1.5 MPa, preferably about 0.4 to 1.2 MPa, at a total pressure of about 0.45 to 1.9 MPa, preferably about 0.6 to 1.5 MPa, and at an HSV (weight of feed/weight of catalyst/hour) of about 0.25 hxe2x88x921 to 30 hxe2x88x921, preferably about 1 hxe2x88x921 to 10 hxe2x88x921, and usually 2 hxe2x88x92l to 6 hxe2x88x921.
An examination of the literature shows that recycling certain constituents contained in the effluent from the isomerisation reactor to the inlet t that reactor has been envisaged with a view to minimising the production of secondary products.
As an example, U.S. Pat. Nos. 3,553,276, 3,558,173 and 4,255,606 recommend adding certain compounds to the feed to be treated, in order to reduce the loss due to secondary products.
Thus U.S. Pat. No. 3,553,276 describes an apparatus in which toluene is recycled so that its concentration is kept at double the concentration which would be obtained without that recycling.
U.S. Pat. No. 3,558,173 describes recycling naphthenes containing eight carbon atoms produced by hydrogenation of the corresponding aromatic compounds.
In the description in U.S. Pat. No. 4,255,606, 1% to 10% by weight with respect to the total feed of an aliphatic hydrocarbon containing at least five carbon atoms is introduced into the reaction zone with or without adding toluene. This addition can be accomplished by recycling. The hydrocarbon introduced can also be a precursor of n-pentane.
The isomerisation process of the present invention comprises introducing into the reaction zone, with the feed containing the compounds to be isomerised and the hydrogen required for the reaction, at least one compound with a boiling point of about 80xc2x0 C. to 135xc2x0 C. and more particularly at least one compound selected from the group formed by: paraffins containing eight carbon atoms per molecule, benzene, toluene, and naphthenes containing eight carbon atoms.
At least one compound selected from the group formed by paraffins containing eight carbon atoms per molecule, benzene, toluene, and naphthenes containing eight carbon atoms is added to the feed to be isomerised, in the form of a recycle, in the form of fresh compound or in the form of a recycle and fresh compound, in quantities such that the percentage by weight of compound added with respect to the total feed entering the reactor is normally:
the percentage by weight of paraffins containing eight carbon atoms in the optional case when these are added to the feed is about 0.1% to 10%, preferably about 0.2% to 2%;
the percentage by weight of naphthencs containing eight carbon atoms in the optional case when these are added to the feed is about 0.5% to 15%, preferably about 2% to 10%;
the percentage by weight of toluene in the optional case when it is added to the feed is about 0.01% to 5%, preferably about 0.01% to 3%;
the percentage by weight of benzene in the optional case when it is added to the feed is about 0.01% to 5%, preferably about 0.01% to 1%.
When a plurality of compounds are added, the percentage by weight of total compounds added represents about 0.01% to 20%, normally about 2% to 15%, with respect to the total feed which enters the reactor. In a preferred implementation of the invention, at least one naphthene is added.
In a preferred implementation of the invention, at least two different compounds are introduced into the reaction zone, each with a boiling point of about 80xc2x0 C. to 135xc2x0 C. More particularly, at least one naphthene and at least one paraffin containing eight carbon atoms are introduced. In a further variation, all of the compounds contained in this liquid fraction with boiling points of about 80xc2x0 C. to 135xc2x0 C. are introduced without separating them. These compounds may originate from recycling a liquid fraction leaving the reactor.